1. Technical Field
The invention relates to liquid separating devices for removing a stream of liquid from a black liquor slurry stream of liquid and solids (black liquor). The invention particularly relates to screen slots in an in-line drainer for separating a stream of excess liquor from the stream of black liquor, which comprises liquid typically containing at least some wood chips or fine wood particles in a pulping system.
2. Related Art
In-line drainers for pulping systems, such as disclosed in U.S. Pat. No. 6,451,172, are typically used in the chip feed of a pulping system. A common use for in-line drainers is to return strained liquid from a black liquor slurry stream to a slurry of wood chips and liquor flowing through a high pressure transfer device to a pulping vessel, such as a continuous digester vessel. The in-line drainer removes some of the liquid in the black liquor slurry stream from the low pressure outlet of a high pressure transfer device. The strained liquid removed from the black liquor slurry stream by the in-line drainer may be reused in the pulping system. The recirculated liquid, which is a mixture of wood chips and remaining liquor flowing from the in-line drainer, flows back into the chip flow from a chip bin and moves to the high pressure feeder.
A difficulty with in-line drainers occurs in the removal of the liquid from the black liquor slurry stream by screening. The screening process associated with separating some of the strained liquid from the black liquor slurry stream retains wood chips, wood fines, and wood pins within the liquor flowing through the in-line drainer (which liquid will become recirculated liquid). Screening is typically performed with a cylindrical basket in the in-line drainer. The walls of the basket include slots or apertures so narrow that wood fines and pins cannot pass through them. Conventional screen baskets are fashioned from steel bars oriented in a parallel, horizontal, or inclined fashion at an angle relative to the direction of flow of the black liquor slurry stream so that the liquor passes through the slots while retaining wood particles within the in-line drainer so the wood particles can be removed from the in-line drainer in the recirculated liquid stream. Conventional cylindrical screen baskets are formed of a solid sheet of material with slots through the sheet, as shown in U.S. Pat. No. 6,451,172.
The black liquor slurry stream passing through an in-line drainer typically has a liquid to wood ratio (on a volume basis) of greater than 5 to 1 (meaning at least 5 parts liquid to 1 part solids) resulting in a low concentration of solids in the stream to the screen basket within the in-line drainer. The black liquor slurry stream flowing through the screen basket moves at a high velocity, such as 10 feet per second. As a result of the low solids concentration and high velocity through the screen basket, the solid material, e.g., wood chips, pins, and fines, easily aligns parallel to the slots and passes through the slots or becomes lodged in the slots of the screen basket.
To prevent solids from passing through or becoming lodged in the slots, conventional in-line drainers have helical baffles or helical flights that impart a helical movement to the black liquor slurry stream flowing through the screen basket. The helical movement of the black liquor slurry stream causes the solids to move in a helical path through the screen basket and not to become lodged parallel to the slots.
Another approach to preventing clogging and to encourage the passage of solids through the slots in the screen basket is to align the slots of the screen basket obliquely to the axial direction of elongation of the in-line drainer, and thus obliquely to the direction of flow through the screen basket. This other approach does not suffer a pressure loss in the black liquor slurry stream that is commonly found when using helical baffles and flights. The angle of the screen basket slots relative to the direction of black liquor slurry stream flow through the screen basket ranges from about 0 degrees (perpendicular) to 90 degrees.
Helical baffles and flights, and slots oblique to the flow of the black liquor slurry stream have not completely eliminated the problem of slots becoming clogged with solids, especially with wood fines and pins. Such clogging is a particular problem occurring when the slots are created by cutting into screen baskets formed from metal plates. There remains a long felt need for screen baskets having slots that are less prone to becoming clogged with solids, such as wood chips, fines, and pins. It is to this need and others that the present disclosure is directed.
The terms chips, fines, and pins herein generally refer to comminuted cellulosic fibrous material may such as wood chips, sawdust, grasses such as straw or kenaf, and agricultural waste such as bagasse and recycled paper. The in-line drainers disclosed herein are applicable to liquid separators for feed systems of both continuous and batch digesters, and also applicable to feeding several continuous digesters or one or more discontinuous or batch digesters.